As a hydraulic driving unit mounted on a construction machine to perform combined operations of plural hydraulic cylinders, there is known, for example, the hydraulic driving unit disclosed in JP 2000-337307A. This hydraulic driving unit is mounted on a hydraulic excavator. FIG. 11 is a hydraulic circuit diagram showing the construction of an essential part of the hydraulic driving unit disclosed in JP 2000-337307A, and FIG. 12 is a side view illustrating a hydraulic excavator on which the hydraulic driving unit shown in FIG. 11 is arranged.
The hydraulic excavator illustrated in FIG. 12 is provided with a travel base 1, a revolving superstructure 2 arranged on the travel base 1, a boom 3 mounted swingably in a vertical direction on the revolving superstructure 2, an arm 4 mounted swingably in a vertical direction on the boom 3, and a bucket 5 mounted turnably in a vertical direction on the arm 4. The boom 3, arm 4 and bucket 5 make up front attachments. The hydraulic excavator is also provided with a boom cylinder 6 which constitutes a first hydraulic cylinder for driving the boom 3, an arm cylinder 7 which constitutes a second hydraulic cylinder for driving the arm 4, and a bucket cylinder 8 for driving the bucket 5.
FIG. 11 shows a center-bypass hydraulic driving unit for driving the boom cylinder 6 and arm cylinder 7 in the above-mentioned hydraulic driving units suitable for arrangement on hydraulic excavators.
As shown in FIG. 11, the boom cylinder 6 is provided with a bottom chamber 6a and a rod chamber 6b. By supplying pressure oil to the bottom chamber 6a, the boom cylinder 6 is caused to extend to perform boom raising. By supplying pressure oil to the rod chamber 6b, on the other hand, the boom cylinder 6 is caused to retract to perform boom lowering. The arm cylinder 7 is also provided with a bottom chamber 7a and rod chamber 7b. By supplying pressure oil to the bottom chamber 8a, arm crowding is performed. By supplying pressure oil to the rod chamber 7b, on the other hand, arm dumping is performed.
The hydraulic driving unit which includes these arm cylinder 6 and arm cylinder 7 is provided with an engine 20, a main hydraulic pump 21 driven by the engine 20, a boom-related, directional control valve 23 as a first directional control valve for controlling a flow of pressure oil to be supplied from the main hydraulic pump 21 to the boom cylinder 6, an arm-related, directional control valve 24 as a second directional control valve for controlling a flow of pressure oil to be supplied from the main hydraulic pump 21 to the arm cylinder 7, a boom control device 25 as a first control device for selectively controlling the boom-related, directional control valve 23, an arm control device 26 as a second control device for selectively controlling the arm-related, directional control valve 24, and a pilot pump 22 driven by the engine 20.
The boom-related, directional control valve 23 is arranged on a line 28 extending to a delivery line of the main hydraulic pump 21, while the arm-related, directional control valve 24 is arranged on a line 27 extending to the above-mentioned delivery line.
The boom-related, directional control valve 23 and the bottom chamber 6a of the boom cylinder 6 are connected via a main line 29a, while the boom-related, directional control valve 23 and the rod chamber 6b of the boom cylinder 6 are connected via a main line 29b. Similarly, the arm-related, directional control valve 24 and the bottom chamber 7a of the arm cylinder 7 are connected via a main line 30a, while the arm-related, directional control valve 24 and the rod chamber 7b of the arm cylinder 7 are connected via a main line 30b. 
The boom control device 25 is connected to the pilot pump 22. A pilot pressure produced as a result of its operation is supplied via one of pilot lines 25a,25b to a corresponding control compartment of the boom-related, directional control valve 23 such that the boom-related, directional control device 23 is changed over into the left position or the right position as viewed in FIG. 11.
Similarly, the arm control device 26 is also connected to the pilot pump 22. A pilot pressure produced as a result of its operation is supplied via one of pilot lines 26a,26b to a corresponding control compartment of the arm-related, directional control valve 24 such that the arm-related, directional control device 24 is changed over into the left position or the right position as viewed in FIG. 11.
In the hydraulic excavator provided with the hydraulic driving unit constructed as described above, the boom control device 25 shown in FIG. 11 is operated upon performing digging or the like of earth, and a pilot pressure is hence produced, for example, in the pilot line 25a. When the boom-related, directional control valve 23 is changed over into the left position as viewed in FIG. 11, the pressure oil delivered from the main hydraulic pump 21 is supplied to the bottom chamber 6a of the boom cylinder 6 via the line 28, the boom-related, directional control valve 23 and the main line 29a, while the pressure oil in the rod chamber 6b is caused to return to a reservoir 43 via the main line 29b and the boom-related, directional control valve 23. As a result, the boom cylinder 6 extends as indicated by arrow 13 in FIG. 12 so that the boom 3 is swung as indicated by arrow 12 in FIG. 12 to perform boom raising.
Concurrently with this boom raising operation, the arm control device 26 is also operated and a pilot pressure is hence produced, for example, in the pilot line 26a. when the arm-related, directional control valve 24 is changed over into the left position as viewed in FIG. 11, the pressure oil delivered from the main hydraulic pump 21 is supplied to the bottom chamber 7a of the arm cylinder 7 via the line 27, the arm-related, directional control valve 24 and the main line 30a, while the pressure oil in the rod chamber 7b is caused to return to the reservoir 43 via the main line 30b and the arm-related, directional control valve 24. As a result, the arm cylinder 7 extends as indicated by arrow 9 in FIG. 12 so that the arm 4 is swung as indicated by arrow 11 in FIG. 12 to perform arm crowding.
When an unillustrated bucket control device is also operated concurrently with such a boom raising and arm crowding operation to change over a bucket-related, directional control valve such that the bucket cylinder 8 illustrated in FIG. 12 is caused to extend in the direction of arrow 10 in FIG. 12, the bucket 5 is caused to turn in the direction of arrow 11 to perform earth digging work or the like as desired.
FIG. 13 contains characteristic diagrams illustrating pilot pressure characteristics and cylinder pressure characteristics in the above-described combined operation. In the lower diagram of FIG. 13, time lengths of digging work are plotted along abscissas, and pilot pressures produced by the control device are plotted along ordinates. A broken line 31 in the lower diagram of FIG. 13 indicates pilot pressures produced by the arm control device 26 and to be supplied to the pilot line 26a, while a solid line 32 in the lower diagram of FIG. 13 designates pilot pressures produced by the boom control device 25 and to be supplied to the pilot line 25a, that is, pilot pressures upon boom raising. T1, T2 and T3 indicate time points at which boom raising operations were performed, respectively.
In the upper diagram of FIG. 13, on the other hand, time lengths of digging work are plotted along abscissas, and load pressures produced in the hydraulic cylinders 6,7, in other words, cylinder pressures are plotted along ordinates. A broken line 33 in the upper diagram of FIG. 13 indicates bottom pressures produced in the bottom chamber 7a of the arm cylinder 7, that is, arm cylinder bottom pressures, while a solid line 34 designates rod pressures produced in the rod chamber 6b of the boom cylinder 6, that is, boom cylinder rod pressures. When such a combined operation of boom raising and arm crowding is performed, force in the direction of arrow 12 in FIG. 12 is transmitted to the boom 3 by counterforce produced when the bucket 5 digs earth. As a consequence, the boom cylinder 6 tends to be pulled in the direction of arrow 13 in FIG. 12, and as indicated by the boom rod pressure 34 in the upper diagram of FIG. 13, a high pressure is produced in the rod chamber 6b of the boom cylinder 6.
In the above-described conventional art, earth digging work or the like can be performed without a problem by combined operations of boom raising and arm crowding. Nonetheless, it is desired to achieve more efficient work.
The present inventors' attention was attracted to the current situation that the pressure oil in the rod chamber 6b of the first hydraulic cylinder as the boom cylinder 6 had been drained directly to the reservoir 43 upon performing the above-described combined operation of boom raising and arm crowding, namely when pressure oil was supplied to both of the bottom chambers 6a,7a of the first hydraulic cylinder as the boom cylinder 6 and the second hydraulic cylinder as the arm cylinder 7 and as a consequence, an operation which would lead to development of a higher rod pressure in the first hydraulic cylinder as the boom cylinder 6 was performed.
The present invention has been completed in view of the above-described situation of the conventional art, and as an object, has the provision of a hydraulic driving unit which makes it possible to effectively use the pressure oil in the rod chamber of the first hydraulic cylinder when the bottom pressure of the second hydraulic cylinder becomes high during a combined operation performed by supplying pressure oil to the respective bottom chambers of the first hydraulic cylinder and second hydraulic cylinder although the pressure oil in the rod chamber of the first hydraulic cylinder has heretofore been drained into the reservoir.